New source of neutrinos from the Sun spotted in a first by Borexino collaboration

In a remarkable experiment designed to bring answers to one of the key mysteries about our Sun, physicists have reported finding evidence for a new source of neutrinos that has been spotted for the first time.

Neutrinos are tiny subatomic particles that interact only via the weak force and gravity. What makes neutrinos unique (and a complex subject to study) is, they are electrically neutral and have a mass so little that it was long thought to be zero. The mass of a neutrino is far smaller than any of the other known particles that make up atoms.

Solar neutrinos, or neutrinos from the sun, are all around us and passing through us at any given time. Scientists have estimated that 100 billion solar neutrinos pass through just our thumbnail every passing second. Neutrons are of the products in the main reaction that powers the sun and other stars – nuclear fusion.

The Sun, captured by NASA's Solar Dynamics Observatory, pointing to "active regions" on its surface where there are bright spots and illuminated arcs, 20 April 2015. Image credit: NASA/SDO

Stars are powered for most of their lives by nuclear fusion in its core. The main kind of nuclear fusion that happens in a star's core is protons (charged hydrogen molecules) being converted to helium. That said, depending on the mass of the star itself, this conversion can happen in different ways.

In the cores of stars like the Sun, the bulk of energy produced (99%) comes from a process called the proton-proton chain reaction (pp). This process creates helium from the fusion of two protons, releasing a positron and a neutrino in the process. The reaction releases an enormous amount of energy in the form of gamma rays.

Photomultiplier tubes on the inside surface of the partly-submerged Kamiokande Neutrino Observatory in Japan. Image: Kamiokande Neutrino Observatory

However, there has been little proof to explain where the missing energy (the remaining 1%) comes from. Physicists have theorised that it may be produced in another set of nuclear fusion reactions called the CNO chain reactions.

The CNO cycle uses carbon, nitrogen, and oxygen as intermediaries to ultimately produce helium, just like in the pp chain. However, there has been no experimental proof of neutrinos from this reaction until now.

In stars that are much heavier than our Sun, the CNO cycle is the main source of energy.

Scientists had already detected neutrinos in 1956, and established a firm link to the most prevalent fusion process, the pp chain, by 2001.

But only now, in a remarkable first, have neutrinos from the second set of reactions – the CNO cycle – been spotted. Researchers from the Borexino experiment have been working to measure low-energy solar neutrinos – a central feature in the story of the missing solar neutrinos – since 2000.

The Borexino team announced their findings on 23 June at the Neutrino 2020 virtual meeting, concluding that the "quest (for) the CNO neutrinos has finally produced the first observation of the signal," in their presentation.

The study is still awaiting peer review before the announcement is made official.

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